This invention relates to an aerofoil structure and method of manufacturing an aerofoil structure, and particularly but not exclusively to a substantially hollow aerofoil structure which is superplastically formed.
It is known to manufacture hollow metallic aerofoils for example to be used as blades in a jet engine, and in particular fan blades for a turbomachine, by superplastic forming and diffusion bonding metallic panels, the panels forming pressure and suction surfaces of the blade. These blades are generally referred to as wide-chord fan blades. The metallic panels may include elementary metal, metal alloys and metal matrix composites. At least one of the metallic panels must be capable of superplastic extensions. In one known process the surfaces of the panels to be joined are cleaned, and at least one surface of one or more of the panels is coated in preselected areas with a stop-off material to prevent diffusion bonding. The panels are arranged in a stack and the edges of the panels are welded together, except where a pipe is welded to the panels, to form an assembly. The pipe enables a vacuum, or inert gas pressure, to be applied to the interior of the assembly. The assembly is placed in an autoclave and heated so as to “bake out” the binder from the material to prevent diffusion bonding. The assembly is then evacuated, using the pipe, and the pipe is sealed. The sealed assembly is placed in a pressure vessel and is heated and pressed to diffusion bond the panels together to form an integral structure. Diffusion bonding occurs when two mating surfaces are pressed together under temperature, time and pressure conditions that allow atom interchange across the interface. The first pipe is removed and a second pipe is fitted to the diffusion bonded assembly at the position where the first pipe was located. The integral structure is located between appropriately shaped dies and is placed within a rig. The integral structure and dies are heated and pressurised fluid is supplied through the second pipe into the interior of the integral structure to cause at least one of the panels to be superplastically formed to produce an article matching the shape of the dies.
In addition to the hollow structure just described, it is also known to insert a membrane 2 between the metallic panels 4, 6 prior to the above described process (see FIG. 1 for example). The location of diffusion bonds between the membrane and the adjacent panels can be controlled by applying the stop-off material to preselected areas on each side of the membrane (or respective panels). When the aerofoil is subsequently expanded, the membrane adheres to the panels where the diffusion bond is allowed to form and thereby provides an internal structure.
FIG. 1 illustrates a known method of manufacturing an aerofoil structure. In this method a metallic sheet 2, made from, for example titanium, is provided for forming the aerofoil structure. The sheet 2 is forged so as to produce two sections 4 of the sheet which are thicker than the main body of the sheet 2. This may be achieved by upset forging, wherein the length of the sheet is reduced in order to obtain the desired increase in cross-section or by drawing the sheet so as to increase the length and reduce the thickness of the main body of the sheet. The sheet 2 may be machine finished prior to the forging and/or splitting/dividing process.
As illustrated in FIG. 1(c) the forged sheet 2 is divided along an inclined plane 6 extending in a span-wise direction, so as to produce two substantially identical panels 8 and 10. The panels 8 and 10 need not be identical and alternative configurations may be used in order to provide desired properties for the resulting aerofoil e.g. the panels may be of differing thickness. The sheet 2 may be divided into the two panels 8 and 10 by way of any known technique which may be suitable for cutting the required width of the aerofoil, for example by using a band saw. To facilitate the cutting process, channels may be machined inboard of the elements 4 to allow the band saw to enter the workpiece, as described in patent application GB2306353 (see FIG. 2). The resulting panels 8 and 10 taper from the section 4 to the tip end of the panel.
The two panels 8 and 10 are then assembled so that their uncut exterior surfaces (which have been machine finished) are facing each other. Optionally, a membrane 12 may be positioned between the panels 8 and 10. As previously described, the assembly may then be diffusion bonded and superplastically formed in order to produce the desired external shape of the aerofoil. When joined, the sections 4 combine to form the root of the aerofoil which serves in use to attach the aerofoil, for example, to the hub of the rotor.
This method of manufacturing an aerofoil structure has certain disadvantages attributable to the forging process which is necessary in order to obtain the sections 4 that form the root of the aerofoil once the two panels 8 and 10 are assembled. In particular the thickness of the root is limited by the forging process such that it is often not possible to produce a root with the required thickness for the application. Furthermore, since the root and aerofoil structures experience different working loads and environments, it is desirable for these structures to have different material properties. This is not possible in the prior art method. GB2306353 does disclose a further step for increasing the thickness of the root, where additional blocks are joined to the root element. However the root element still comprises the material of the aerofoil structure and thus it is still not possible to obtain substantially different properties for the root and aerofoil structures.
EP1605135 discloses a method of making and joining an aerofoil and root wherein an aerofoil structure is prefabricated and then joined to a root element by means of electron beam welding or linear friction welding. This method requires the root element and also the joining surface of the aerofoil structure to be machined prior to joining. This therefore adds additional steps to the process which increases costs and manufacturing time.